Phosphoinositides, in both their water-soluble and lipid forms, have a prominent role in cellular signal-transduction events. Important events are the generation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and its regulation of intracellular Ca2+ homeostasis (1) and the 3-phosphorylated inositol lipid products of phosphatidylinositol (PI3 kinase) (2), with diverse roles in mitogenesis, apoptosis and vesicle trafficking. Phosphatidylinositol 4,5-bisphosphate (PtdIns (4,5)P2), the major source of these two signalling systems, is not merely a precursor for the above signal transduction pathways but plays in itself significant roles in vesicle trafficking, exocytosis, cytoskeletal rearrangements and regulation of ion channels (3). In the last decade there has also been a growing appreciation that highly phosphorylated inositol polyphosphates, distant derivatives of the Ins(1,4,5)P3 second messenger, play a role in signal-transduction and cellular regulation (4-6). Perhaps the most exciting new vista that has opened concerns the role of diester derivatives of both inositol pentakis- and hexakisphosphates (InsP5 and InsP6). The pyrophosphate derivatives of InsP6 diphosphoinositol pentakisphosphate, and bis-(diphospho)inositol tetrakisphosphate are commonly referred to as ‘InsP7’ and ‘InsP8’. These inositol pyrophosphate derivatives rapidly turnover and are estimated to have similar free energy of hydrolysis as ATP (4). A striking consequence of this high-energy phosphate group is the ability of InsP7 to directly phosphorylate a subset of proteins in an ATP- and enzyme-independent manner (7). The variety of cellular responses, apparently controlled by these molecules (4,8) may be facilitated by the differential intracellular distribution of the kinases that make them (9). The concentrations of inositol pyrophosphates can be dynamically regulated during key cellular events, underscoring their importance for cell function. For example, InsP7 levels change during cell cycle progression (10) and InsP7 regulates cyclin/CDK complexes (11) whereas InsP8 increases acutely in response to cellular stress (8). However, recent work has also demonstrated a role for InsP6 as an enzymatic co-factor and so by analogy, it is possible that even under non-stimulatory conditions, InsP7 could be an important regulatory molecule.
Phosphoinositides are also key regulators of the insulin secreting pancreatic β-cell (12). These cells are critical players in blood glucose homeostasis and act by coupling increases in the concentration of glucose and other circulatory or neuronal-derived regulators, to the exocytosis of insulin. The highly phosphorylated InsP6 is particularly interesting as it has been shown to activate voltage-dependent L-type Ca2+ channels (13), exocytosis (14,15) and dynamin-mediated endocytosis (16), all key processes in insulin secretion. A role for InsP7 in the β-cell has not yet been determined. However, given the suggested involvement of inositol pyrophosphates in vesicle trafficking (4), the critical nature of such trafficking events for the process of insulin exocytosis and the high β-cell concentration of InsP6 (13), the immediate precursor of InsP7, we postulated that inositol pyrophosphates may play a significant role in the β-cell. We now demonstrate a novel role for InsP7 in the regulation of insulin exocytosis.